Although Mark Twain was not himself an environmentalist, he was deeply sensitive to the interdependence of humankind and the natural world, to the conditions, often difficult, under which we inhabit the natural world, and to its ultimate indifference to our desires -- issues that continue to preoccupy environmentalists and ecocritics, whether they are inclined toward public policy or philosophy.  This article argues that the Mississippi River is touchstone for Twain's relationship to the natural world and that Life on the Mississippi, a record of his return to the river in the aftermath of the great flood of 1882, demonstrates the power and meaning of its indecipherability in Twain's imagination.